Texas has enacted a law that will make border crossings illegal and punishable with jail time, one of the toughest immigration laws passed by any US state in modern times.
Governor Greg Abbott, a Republican, said it would "stop the tidal wave of illegal entry into Texas".
Immigration advocates have said the law will lead to racial profiling.
It comes amid rising illegal migration and public concern over US President Joe Biden's handling of the border.
The new Texas measure is so controversial, in part, because US courts have previously ruled that only the federal government can enforce immigration laws.
Crossing the border illegally is already a federal crime, but violations are currently handled as civil cases by the immigration court system.
SB4, which is due to come into effect in March, passed both houses of Texas' Republican-led legislature in November.
In practice, the law allows local and state police officers to stop and arrest anyone suspected of having crossed the border illegally, except in schools and hospitals.
Punishments range from misdemeanours to felonies that can lead to jail time or fines of up to $2,000 (£1,580).
A judge can also order that those arrested be sent back across the border into Mexico, although it is unclear how Texas authorities plan to enforce that provision.
Penalties for illegal re-entry could go up to 20 years in jail, depending on a migrant's immigration and criminal history.
In November, Texas Lieutenant Governor Dan Patrick said the law would "keep Texas safe" and that it was "the strongest border security bill Texas has ever passed".
SB4, however, is almost certain to face legal challenges from immigration advocates. The bill has also been criticised by Democratic lawmakers and Mexico's government.
In October, Sarah Mehta, senior border policy counsel at the American Civil Liberties Union, said that "laws like these result in the unlawful racial profiling, arrest and detention of both US citizens and immigrants".
White House spokesperson Angelo Fernández Hernández said that "generally speaking, the federal government - not individual states - is charged with determining how and when to remove noncitizens for violating immigration laws".
The BBC has contacted the justice department - which would be responsible for filing any federal legal action against Texas - for comment.
Border and immigration issues have become a political headache for President Biden.
During the 2022 fiscal year that ended in September, a total of 2.4 million migrant "encounters" were registered at the border, a record high.
Republicans lawmakers have sought to use US military aid to Ukraine as leverage to secure policies to crack down on illegal immigration.
